Allow using special char

[mckelvin]

I was trying to write a replace for a changelog in the follow format.

    # https://keepachangelog.com/en/1.0.0/

    ## [Unreleased]
    ### Added
    - Foobar

    ## [0.0.1] - 2014-05-31
    ### Added
    - This CHANGELOG file to hopefully serve as an evolving example of a
      standardized open source project CHANGELOG.
    """))

but later I found the # is treated as the default comment prefix by the ConfigParser.

This PR is trying to workaround the issue by substituting the # using {#}. Please check the test case for details.

--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 # https://keepachangelog.com/en/1.0.0/

 ## [Unreleased]
+
+## [1.6.0] - 2019-03-19
 ### Added
 - Foobar

[ekohl]

Thanks!